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Spiky Probe on NASA Mars Lander Raises Vapor Quandary

09.04.08

Phoenix inserted the four needles of its thermal and conductivity probe into Martian soil during the 98th Martian day, or sol, of the mission and left it in place until Sol 99 (Sept. 4, 2008).
Image credit: NASA/JPL-Caltech/University of Arizona/Texas A&M University
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TUCSON, Ariz. -- A fork-like conductivity probe has sensed humidity
rising and falling beside NASA's Phoenix Mars Lander, but when stuck
into the ground, its measurements so far indicate soil that is thoroughly
and perplexingly dry.

"If you have water vapor in the air, every surface exposed to that air will
have water molecules adhere to it that are somewhat mobile, even at temperatures
well below freezing," said Aaron Zent of NASA Ames Research Center, Moffett
Field, Calif., lead scientist for Phoenix's thermal and electroconductivity probe.

In below-freezing permafrost terrains on Earth, that thin layer of unfrozen
water molecules on soil particles can grow thick enough to support microbial
life. One goal for building the conductivity probe and sending it to Mars
has been to see whether the permafrost terrain of the Martian arctic has
detectable thin films of unfrozen water on soil particles. By gauging how
electricity moves through the soil from one prong to another, the probe can
detect films of water barely more than one molecule thick.

"Phoenix has other tools to find clues about whether water ice at the site
has melted in the past, such as identifying minerals in the soil and observing
soil particles with microscopes. The conductivity probe is our main tool for
checking for present-day soil moisture," said Phoenix Project Scientist Leslie
Tamppari of NASA's Jet Propulsion Laboratory, Pasadena, Calif.

Preliminary results from the latest insertion of the probe's four needles into
the ground, on Wednesday and Thursday, match results from the three similar
insertions in the three months since landing.

"All the measurements we've made so far are consistent with extremely dry soil,"
Zent said. "There are no indications of thin films of moisture, and this is puzzling."

Three other sets of observations by Phoenix, in addition to the terrestrial
permafrost analogy, give reasons for expecting to find thin-film moisture in the soil.

One is the conductivity probe's own measurements of relative humidity when the
probe is held up in the air. "The relative humidity transitions from near zero
to near 100 percent with every day-night cycle, which suggests there's a lot
of moisture moving in and out of the soil," Zent said.

Another is Phoenix's confirmation of a hard layer containing water-ice about
5 centimeters (2 inches) or so beneath the surface.

Also, handling the site's soil with the scoop on Phoenix's robotic arm and
observing the disturbed soil show that it has clumping cohesiveness when first
scooped up and that this cohesiveness decreases after the scooped soil sits
exposed to air for a day or two. One possible explanation for those observations
could be thin-film moisture in the ground.

The Phoenix team is laying plans for a variation on the experiment of inserting
the conductivity probe into the soil. The four successful insertions so far have
all been into an undisturbed soil surface. The planned variation is to scoop away
some soil first, so the inserted needles will reach closer to the subsurface ice layer.

"There should be some amount of unfrozen water attached to the surface of soil
particles above the ice," Zent said. "It may be too little to detect, but we haven't
finished looking yet."

The thermal and electroconductivity probe, built by Decagon Devices Inc., Pullman,
Wash., is mounted on Phoenix's robotic arm. The probe is part of the lander's Microscopy,
Electrochemistry and Conductivity instrument suite.

The Phoenix mission is led by Peter Smith at the University of Arizona with project
management at NASA's Jet Propulsion Laboratory in Pasadena, Calif., and development
partnership at Lockheed Martin in Denver. International contributions come from the
Canadian Space Agency; the University of Neuchatel, Switzerland; the universities of
Copenhagen and Aarhus in Denmark; the Max Planck Institute in Germany; and the Finnish Meteorological Institute.